Field
This disclosure relates generally to integrated circuit memories and more specifically to sector retirement for a split-gate memory.
Related Art
Some data processing systems may include various types of memory. For example, a volatile memory type that may be used is static random access memory (SRAM), and a non-volatile memory type that may be used is flash memory. Volatile memory types lose their stored states when powered down while non-volatile memory types retain their stored states. The different memory types have advantages and disadvantages. For example, SRAM has faster access times than most non-volatile memory types. Also, some non-volatile memory types have relatively poor program/erase cycling endurance. A thin film storage (TFS) non-volatile memory uses, for example, nanocrystals or silicon-oxide-nitride-oxide-silicon (SONOS) as the storage medium instead of a floating gate. One type of TFS non-volatile memory cell has both a control gate and a select gate and is commonly known as a split-gate memory cell. To access a split-gate memory cell both the control gate word line and the select gate word line of the cell are asserted.
Some areas of a memory array may be accessed more frequently than others, and thus will wear out faster. Memory life can be extended if the areas that wear out are “retired”, or removed from service. In a flash memory, unselected, or retired array portions are biased at zero volts, or ground, in read mode. However, in one type of TFS split-gate memory design, when a sector is retired the unselected drivers of a retired sector are biased at a voltage greater than zero volts, meaning that certain defects in the sector can create an undesirable current path. The current path can lower the actual read voltage on the shared supply, thus lowering a supply voltage that is shared between the retired and active array portions. The lowered read voltage would result in a threshold voltage (VT) margin loss, and possibly read failures.
Therefore, a need exists for a non-volatile memory system that solves the above problems.